Thoughtful question.

When I was 9 or 10, I went by myself to a decrepit old movie house in my Brooklyn neighborhood to see "Son of Frankenstein" (1939). The scene when the monster appears in the little kid's bedroom absolutely paralyzed me--I had a real hard time getting to sleep after that.
Possibly the most creative, scariest, nauseating scene ever filmed is in John Carpenter's "The Thing" (1982), when Dr. Copper applies CPR to Norris and unleashes his head from his gut:
https://www.youtube.com/watch?v=JjIXwkX1e48